Overview | Manufacturing | Technical Info.
A Ceramic Filtration Primer |
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What are ceramic water filters?: The ceramic filters described in this article are made of diatomaceous earth (DE) or diatomite, often mixed with other materials such as clays to aid in the forming process. Diatomite is a highly porous, whitish colored material that is mined from great deposits worldwide. Deposits were formed from the settling of the "skeletons" of a type of fresh or saltwater algae over the eons. Diatom skeletons are composed primarily of silica, and form intricate geometric shapes that are 60-90 percent air, with intra-particle pores often smaller than 1 micron. Combined and sintered (fired or heated in a kiln to the point the silica begins to melt), diatomite tubes are usually 50-70 percent air, are relatively inert chemically and show little particle shedding into the product water or other fluid. Filters can be made with other combustible media blended with the diatomite and clay. The media additive will burn out during sintering, leaving voids to increase porosity. Silica is hard, yet these filters can be abraded with a fingernail due to the brittle nature of the silica and the thin web of the matrix. There is another type of "ceramic" filter, composed of solid particles of alumina or other refectories, that shouldn't be confused with the sintered diatomite. Tubes made of sintered alumina are less porous than diatomite filters, and so have higher resistance to flow. Alumina tubes are not abraded, but are cleaned by acid washing, burning, or other methods. They are also generally considerably more expensive than sintered DE filters. Diatomite filters are typically formed in the shape of a tube prior to sintering. The forming has traditionally been done in a process called slip-casting, dating back over a century. A slurry is poured into a plaster mold and a green state ceramic deposit builds up in the mold as moisture is wicked away. Once the green ceramic is removed from the mold and thoroughly dry, it is ready to be fired. The mold has to be dried before it can be re-used. Degradation of the mold occurs, limiting its re-use, typically to forty-five castings. A much more modern forming method is extrusion. Much like squeezing toothpaste from a tube, the green ceramic mud is forced through a die with a core that forms a hollow cylinder. Pore size is controlled, in part, by the temperature and time
that the ceramic tube is fired. Higher firing temperatures can
create a more open structure, within limits. How they work: Flow. In water filter applications, flow direction is usually from outside to inside so the forces on the ceramic are compressive rather than tensile. Such radial flow results in 100% product water recovery with no reject or waste water as in cross-flow membrane filtration. Ceramic membranes do exist, but again, they are typically of the sintered alumina type not addressed in this article. Wall thickness for common sintered DE tubes is 0.250-0.500 inches, with resistance to flow proportional to the wall thickness for a given media. Modern extruded ceramics can be produced with thinner walls than traditional slip-cast ceramics. Sintered diatomite filters have relatively high resistance to flow compared to non-ceramic media due to the fineness of the pore structure, a relatively thick wall, and limited available surface area in a cartridge. Maximum flow rate will be achieved after the ceramic is fully wetted or saturated. Flow decline is caused principally by particles in sizes from 0.2 to 1.0 micron sticking on the 0.005-inch outer surface of the ceramic media. This material can be cleaned off for flow recovery. It has not always been found beneficial to place a series of pre-filters in descending porosity upstream of ceramics. Larger particles tend to rest on the exterior surface of the ceramic, not fully blocking water passage. True clogging of a ceramic occurs with smaller particles. Thus, simpler, easier to maintain systems can be operated more economically without 20, 5, and 1-micron pre-filters, letting the ceramic take on the raw water of unknown quality as the first line of defense. Cleaning time is usually less than one minute and might be compared to scrubbing dried-on food from a plate after a meal. Filtration. Looking through a microscope, most water filter media resembles a sponge more than a window screen. Clogging particles - mostly soil mineral particles - are either stopped near the surface of the filter or migrate deep into the media and stop there. If particles like bacteria are stopped simply because they are too big to fit through a hole, we call that "sieving". When a particle or molecule sticks to the filter media like a ball bearing sticks to a magnet, this is called "adsorption". All filter media rely on some combination of sieving and adsorption to remove "stuff" from water. Some filters (ceramic filters, for example) are heavy on the sieving, others (carbon block filters) are heavy on the adsorption. Actual retention of particles smaller than the median pore diameter of the diatomite filter media is achieved because of its depth and tortuous path, as well as surface adhesion and/or adsorptive properties. Picture a golf ball easily being thrown through a chain-link fence as a two-dimensional somewhat ineffective barrier. The same size openings in multiple layers of chain-link fence, not perfectly aligned, (a three-dimensional barrier), present an impenetrable barrier, no matter how forcefully the golf ball is thrown. Thus, one can readily see the critical importance of wall thickness and an irregular pathway, allowing media, for example, with a median pore diameter of 1.9-microns to provide effective retention of 0.2-micron-sized particles. Also, DE filters are inert and do not add chemicals nor do they shed particles into the water.
Applications: Ceramic filtration is one of the preferred technologies in emergency preparedness and disaster relief situations, where the challenge is to provide potable quality water at a low cost without reliance on water pressure, electricity, or even chemicals. Siphon filters are common in such situations. Similarly, ceramic filters have been used for decades in developing countries for cleaning daily drinking water in gravity-fed systems, where water flows from an upper reservoir through the ceramic and into a lower collection container. Carbon or other media is often placed in the interior of a ceramic cylinder to assist with removing chemicals, tastes and odors. The tight porosity of a ceramic serves as a flow restrictor, increasing contact times and can improve the effectiveness of such media. Prefiltration and the prevention of downstream media occlusion or blinding is also achieved. Ceramic filters are used in gravity-fed, siphon, pressurized point-of-use and end-of-tap systems, and in outdoor backpacking hand-pump units. In some industrial/commercial applications (when combined with proper end-caps e.g. stainless steel), ceramics can withstand very harsh filtration conditions, as they are ozone resistant, chemical resistant, inert, and heat resistant, having already been exposed to 900 - 1100 degrees Celsius for several hours during sintering. Filters can be produced that can even be autoclaved or boiled for sterilization. With modern extrusion technology and new, more consistent
pore size formulations, modern, innovative ceramics are finding
acceptance for industrial, commercial and whole-house point-of-entry
(POE) applications, especially when greater surface area for
higher flow is required without sacrificing retention performance.
Extrusion allows for longer lengths, larger diameters, and thinner
walls than traditional slip-casting. Consequently, the only 4"
x 20" sintered DE ceramic cartridge filter for industry-standard
20" "Big Blue" housings is produced by extrusion. Ceramic Advantages:
Perceived Limitations:
Things to be aware of when comparing ceramic filters: Certainly all ceramics are not created equal, so how does one judge a good from a not so good ceramic? First, one has to determine the true requirements for a given application.Pore size, retention, overall permeability, flow restriction, pressure drop, uniformity of structure, wall thickness, taper (of slip-cast filters), and integrity of end-cap seals (assembly and materials) should all be considered. Some ceramic filters are powdery, fragile, and produced from differing grades and sources of raw materials. Mined diatomite and clay variances will also contribute to minor coloration differences in some ceramics. Coloration is not an appropriate means of assessing the quality of a ceramic filter. Kiln temperature inconsistency or excessive temperatures in firing will adversely affect filter consistency and performance. Subsequent failure to perform adequate quality inspections can result in variable results. A product tends to only be as good as the company that makes it. Assess the manufacturer's ability to provide custom configurations, shapes, wall thickness, length, and end-caps.Slip-cast parts often vary in wall thickness by 0.070 inches, rendering minimum wall thickness measurements with an external caliper gauge ineffective and unreliable. Secondary machining operations can correct such inherit limitations of slip-casting, but at added cost. Extrusion allows for the production of longer parts with no taper as is required for removal of cast parts from molds. Roundness, taper, concentricity, wall thickness, and curvature can all be controlled more consistently with extrusion. As ceramics are non-compressive, perpendicularity of the end cut and other dimensional tolerances of a ceramic tube are vital.Surface treatment of ceramics by abrasion can provide a more aesthetic filter, as well as maximize initial performance. Compare if the parts have been flushed at the factory, or if the burden is placed on the user to flush possible water-soluble minerals from the ceramic before initial use.Were bubble point, mercury porosimetry, PADDS (pressurized air defect detection system), or flow rate (with water and/or air) tests performed? If so, at what sampling intervals or 100% inspection? Packaging is very important to assure receipt of undamaged parts. Micron rating comparison is problematic, and not just with ceramic filters. Is one referring to nominal or absolute retention? Absolute has been defined in a number of ways, from a 2 log (99%) reduction, to a 4 log (99.99%) reduction, to even a 7 log reduction (as per Health Industry Manufacturer's Association (HIMA) definition for pharmaceutical filter media). "7 log" removes 100,000 times more organisms than "2 log". A 1 log (98%) reduction against an influent challenge of 500,000 organisms, still leaves 10,000 bacteria per 100ml. The newest standards proposed by AquaEuropa call for a 99.8% efficacy at any given retention level claimed for particulate filters down to 1-micron in size. After establishing agreement as to the definition, was the filter tested at maximum flow rate? At minimum wall thickness? With a bacterial or solid particle challenge? What size and type of organism was used? E. coli., klebsiella terrigena, or brevundimonas diminuta? How were the bacteria fed and grown? Was testing done with mono-dispersed organisms (reduced clumping or agglomeration)? What was the concentration of the contaminant in the influent challenge water? Was the testing done with media that had been exposed to field surface water to use up adsorptive capacity, or simply aged with de-chlorinated tapwater? Hence, it is not uncommon for a product listed as a 0.3-micron filter to, in fact, remove many times more bacteria than a 0.2-micron filter in side by side, equivalent testing. One way pore structure of a sintered diatomite filter media is measured utilizes a method called "mercury porosimetry," where mercury is forced into a small sample of the media under ever increasing, carefully controlled pressures. The volume of media "intruded" at any given pressure, which correlates to a given pore size, is measured. Mercury porosimetry gives a close-up look at the pore size distribution of the media, which correlates to retention capability and flow rate. More open structure allows higher flow rate, but doesn't stop as many microbes. Mercury porosimetry testing provides much more comprehensive data than traditional bubble-point testing. Overall porosity or permeability is another point of comparison.Durability
should be noted, as an overly soft ceramic might be too easily
abraded, shortening its useful life. Conversely, a very hard
ceramic might be too difficult to clean by abrading. Conclusion: Ceramic technology has evolved over the decades. At their inception, ceramic filters were state-of-the-art in water treatment technology. Over time, alternative technologies were developed, and ceramics produced with decades-old, slip-casting techniques became less popular, particularly in developed countries. Today, through modern manufacturing and testing innovations, ceramics are returning to their former position as truly "state-of-the-art" in the eyes of major manufacturers of water treatment products. A growing body of laboratory data and field experience is proving that many technologies perhaps thought of as competing with ceramics are actually enhanced by ceramics through superior pre-filtration protection of downstream media, enhancing performance efficiency, extending useful life, improving economics, and providing a reliable fail-safe mechanism. As a user-friendly, field-serviceable technology, people in all walks of life can benefit from this increasingly "appropriate" technology. |
